A radiation detector for an X-ray CT apparatus there has conventionally used a xenon gas chamber, a combination of bismuth germanium oxide (BGO single crystal) and a photomultiplier tube, as well as combinations of a Csl:Tl single crystal or CdWO4 single crystal and a photodiode. In recent years, a rare-earth phosphor having good radiation-to-light conversion efficiency has been developed, and a radiation detector using a combination of such a phosphor and a photodiode has already been put into practical use.
A rare-earth phosphor consists of a rare-earth element oxide or a rare-earth element oxysulfide as a base material, to which is added an activator that serves as a luminescent component. As a rare-earth element oxide phosphor, a phosphor including yttrium oxide and gadolinium oxide as a base material, and a phosphor represented by a formula (Gd1-xCex)3Al5-yGayO12 have been proposed in Japanese Patent Laid-open Publication JP-A-3-50991 and in International Patent Laid-open Publication WO99/33934 (PCT/JP98/05806), respectively.
Properties generally required of a scintillator material of the type used in a radiation detector include high luminescence efficiency, short afterglow, and high X-ray stopping power. There are phosphors having high luminescence efficiency among the above-mentioned phosphors, but the afterglow time thereof is relatively long. When the afterglow of the scintillator used in X-ray detectors of an X-ray CT apparatus is large, the acquired information becomes indistinct along the time-axis. Many conventional scintillator materials have a problem with large afterglow. However, the phosphor (Gd1-xCex)3Al5-yGayO12 (0.0005≦x≦0.02, 0<y<5), mentioned in the International Patent Laid-open Publication WO99/33934, has excellent scintillator properties, including both high light emission output and short afterglow.
Although it has been confirmed that (Gd1-xCex)3Al5-yGayO12 material in powder form certainly has good scintillator properties, it has been also found that a new problem occurs when manufacturing a scintillator plate using this powder composition. That is, the luminescence efficiency and the afterglow greatly fluctuate when sintering the scintillator plate, and it is hard to obtain stable properties. Especially, it has been revealed that the afterglow properties are significantly deteriorated by sintering, and the thickness of the sintered body increases as well. A thick sintered body is indispensable for a low cost manufacturing technique, so that there has been a need for a technique for manufacturing a thick sintered body with low afterglow.
An object of the present invention is to provide a phosphor which has a thick plate or block shape that is suitable for mass production, while also having a high luminescence efficiency to X-rays, an extremely low afterglow, and good reproducibility of these properties, by solving the instability of the scintillator properties, which is a drawback of the phosphor of (Gd1-xCex)3Al5-yGayO12 composition.
Another object of the present invention is to obtain a radiation detector with large light emission output and low afterglow by using the above-described phosphor as a scintillator of a radiation detector having a light detector.
Still, another object of the present invention is to provide a tomogram having high resolution and high quality by applying the above-descibed radiation detector to an X-ray CT apparatus.